A motor vehicle and a mower collided on the Baltimore Washington Parkway around 3:45 p.m. Tuesday.
The operator of the mower, who is a contractor with the National Park Service, was taken to Prince George's Hospital Center with life-threatening injuries.
The United States Park Police are investigating the crash. Investigators have not yet determined if the motor vehicle left the roadway or the mower was not in the median at the time of the collision.
The cause of the incident remains under investigation. The BW Parkway was closed for several hour at MD-410 while police investigated the scene.